Fabrication Audit Checklist and Records

An audit is a walk through a shop comparing what is written with what is done, and the records are what make the comparison possible. A shop that runs a good process and keeps poor records fails an audit, and a shop that keeps good records and runs a poor process passes one, which is why the records are part of the process rather than a report on it.

What an Audit Follows

The auditor starts with a finished board or a material lot and follows it backwards through the records to the incoming material. The path is the same path a defect would follow, and a gap in the path is a gap in the traceability.

That is why the records have to be joined rather than complete. A set of perfect records that cannot be connected describes nothing. Our fabrication notes describe the requirements the records are compared against.

The Records That Are Usually Asked For

The material certifications, the stack-up and the drawing revision, the drill and plating parameters, the coupon measurements, the final inspection results and the panel identification. Each is a short record and together they describe the lot.

Where the records are held in different systems, the linkage is the identifier rather than the file. A lot number that appears in all of them is what makes the path followable. Our plating thickness notes describe one of the parameters that is recorded.

Process control is evidenced by the measurements that were taken, the limits they were compared with and the action that followed a result outside a limit. A measurement without a limit is a record of an observation rather than of a control.

The action is the part that is most often missing. A record that shows a result outside the limit and no response is a record that the control did not function. Our board quality notes describe how the acceptance is applied.

Every instrument used for a recorded measurement is calibrated, and the calibration certificate has to cover the date of the measurement and the range used. A certificate that is current and does not include the range is not coverage.

The list of instruments is therefore part of the audit preparation. An instrument that is used and not on the list is an instrument whose calibration is unknown.

The audit follows a change from the request to the implementation, and the evidence is the approval and the revision of the document that was changed. A change that was made and not recorded is found by comparing a record with the current revision.

Where the shop has a manufacturing execution system, the revision is enforced at the station. Where it does not, the removal of superseded copies is the control.

A finding is closed by a change to the process and not by a note. The correction is verified by re-examining the records that the finding came from, and the systemic correction is verified on the next audit.

The distinction between the two matters, because a finding that is closed with a note returns. The useful measure is the number of repeat findings rather than the number of findings.

Acceptance and Its Evidence

A record that identifies the operator, the date and the settings is worth more than a record that identifies only the result. A measurement taken at the wrong point of the process describes the wrong thing, however carefully it is made.

Where the supplier and the user both measure the same property, they should agree on the method before the first delivery. A parameter that is set once and never re verified drifts, and the drift is usually discovered by a defect rather than by the record.

The first article confirms that the setup matches the intent, and it is the cheapest point at which a wrong setup can still be corrected. The cost of verification is small compared with the cost of a field failure, and it is paid at a point where the product can still be corrected.

Documentation exists so that a person who was not present can reproduce the work and reach the same conclusion. The acceptance criteria should be written before the work starts, so that the decision is made by the specification rather than by the person inspecting.

The environment around the process, including temperature, humidity and cleanliness, sets limits on what the process can hold. A result that cannot be reproduced is not a result, and reproducibility should be demonstrated rather than assumed.

Checks Before Release

Sampling is a compromise between cost and confidence, and the sample size should follow from the failure rate that has to be detected. The checks that matter are the ones performed on the product rather than on a sample kept for the purpose, because a coupon that travels with the panel is the only evidence about that panel.

Where two operations share a tolerance, the allocation between them should be explicit rather than left to whichever is measured first. The tooling, the material and the profile form one system, and a change to any of them should be assessed against the other two before it is released.

Verification and Records

The sequence of operations is part of the specification, because a different order produces a different result from the same steps.

Is the audit passed by having documents? It is passed by being able to follow a lot from the material to the finished board through connected records.

Does a measurement need a limit? To be a control it does. A measurement without a limit and a response is an observation.

What does gopcba provide for audits? We provide records joined by a lot identifier rather than complete but separate, parameters compared with stated limits and the response recorded, calibration coverage including the range, change control with the revision enforced at the station, and findings closed by a process change rather than a note.

Points to Confirm at First Article

Where a requirement can be measured, it should be measured at the point of manufacture and recorded against the board or the lot it applies to. Where the process window is narrow, the measurement resolution has to be better than the window, or the data cannot distinguish a good part from a marginal one.